Electrochemical sensors or cells are widely used to determine electrochemically active chemical species in liquid, gas and vapor phases. Such electrochemical sensors can be conveniently classified as galvanic when operated to produce electrical energy and electrolytic when operated at a constant potential via consumption of electrical energy from an external source. Many electrochemical sensors can be operated in either a galvanic or an electrolytic mode.
In an electrochemical sensor, the chemical species to be measured (the "analyte") typically diffuses from the test environment into the sensor housing through an analyte-porous or analyte-permeable membrane to a working electrode (sometimes called a sensing electrode) wherein the analyte chemically reacts. A complementary chemical reaction occurs at a second electrode in the sensor housing known as a counter electrode (or an auxiliary electrode). The electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte gas at the working and counter electrodes.
In general, the electrodes of an electrochemical sensor provide a surface at which an oxidation or a reduction reaction occurs (that is, an electrochemically active surface) to provide a mechanism whereby the ionic conduction of an electrolyte solution in contact with the electrodes is coupled with the electron conduction of each electrode to provide a complete circuit for a current. By definition, the electrode at which an oxidation occurs is the anode, while the electrode at which the "complimentary" reduction occurs is the cathode.
To be useful as an electrochemical sensor, a working and counter electrode combination must be capable of producing an electrical signal that is (1) related to the concentration of the analyte and (2) sufficiently strong to provide a signal-to-noise ratio suitable to distinguish between concentration levels of the analyte over the entire range of interest. In other words, the current flow between the working electrode and the counter electrode must be measurably proportional to the concentration of the analyte over the concentration range of interest.
In addition to a working electrode and a counter electrode, an electrolytic electrochemical sensor often includes a third electrode, commonly referred to as a reference electrode. A reference electrode is used to maintain the working electrode at a known voltage or potential. The reference electrode should be physically and chemically stable in the electrolyte and carry the lowest possible current to maintain a constant potential.
As discussed above, the electrical connection between the working electrode and the counter electrode is maintained through an electrolyte. The primary functions of the electrolyte are: (1) to efficiently carry the ionic current; (2) to solubilize the analyte in its gas phase; (3) to support both the counter and the working electrode reactions; and (4) to form a stable reference potential with the reference electrode. The primary criteria for an electrolyte include the following: (1) electrochemical inertness; (2) ionic conductivity; (3) chemical inertness; (4) temperature stability; (5) low cost; (6) low toxicity; (7) low flammability; and (8) appropriate viscosity.
Electrochemical sensors typically use aqueous electrolytes and porous hydrophobic membranes as electrode supports and as gas diffusion barriers. In other words, such porous membranes perform two functions: (1) acting as a support for an electrochemically active material such as an electrocatalyst; and (2) acting as a diffusion barrier. The diffusion barrier allows diffusion of the analyte in its gas phase into the sensor to contact the electrocatalyst, while effectively retaining the aqueous electrolyte within the interior of the sensor.
The most commonly used aqueous electrolytes incorporate solutions of sulfuric acid, in part, because of their insensitivity to carbon dioxide (CO.sub.2) which is commonly present in test environments. Moreover, sulfuric acid provides an aqueous electrolyte containing a nonvolatile solute. Unfortunately, the use of aqueous electrolytes is restricted by a number of factors, including the range of electrical potentials at which water decomposes and by the high vapor pressure of water. Aqueous electrolytes also have a high dielectric constant and, therefore, can generally dissolve more gas. However, such high gas dissolution rates create a number of measurement distortions.
Most electrochemical toxic sensors employing aqueous electrolyte solutions include porous polytetrafluoroethylene (PTFE) diffusion barrier membranes such as Gore-Tex.RTM. or Zitex.RTM.. These materials provide a generally effective means of fabricating a variety of useful sensors with generally acceptable output characteristics. For example, such sensors exhibit adequate sensitivity, long life (typically at least one year, and up to five years or more) and freedom from liquid leaks over the lifetime of the sensor. Current diffusion barriers made from materials such as Gore-Tex and Zitex generally operate best under conditions in which the pH of the electrolyte solution is less than 7.0. Such diffusion barriers often fail at a pH above 7.0 or if a solute, such as a charge carrier, is added to the electrolyte solution. It is believed that such failure (that is, bulk passage of electrolyte through the membrane) is associated with a reduction of surface tension.
Very little research has been performed with electrochemical systems comprising "non-aqueous" electrolytic solutions. This lack of research may stem, in part, from the unfeasibility of excluding water from such systems during use. Moreover, current diffusion barrier membranes (such as Gore-Tex and Zitex) have little or no effectiveness for retaining non-aqueous liquid electrolytes.
A few non-aqueous electrolyte systems have been attempted in electrochemical sensors with very limited success. For example, in a sensor disclosed in U.S. Pat. No. 4,184,937, a non-aqueous electrolyte comprising primarily propylene carbonate was employed with a Gore-Tex diffusion barrier membrane. Because of certain physiochemical characteristics of propylene carbonate, particularly surface tension, propylene carbonate was retained by the Gore-Tex membrane with limited success. Nonetheless, the electrolyte of that sensor could be made to flow through the diffusion barrier membrane with relative ease. Flow of electrolyte through the diffusion barrier membrane results in failure of an electrochemical sensor for two reasons: (1) bulk loss of electrolyte and (2) filling of the diffusion pores of the membrane with electrolyte, effectively ending its usefulness as an analyte-porous diffusion barrier. Other attempts at using non-aqueous electrolyte solutions, such as disclosed in U.S. Pat. No. 4,522,690, have involved gellation of the non-aqueous electrolyte to prevent loss thereof.
The present inventors have discovered that numerous and significant advantages can be achieved with the use of non-aqueous electrolyte systems. It is, thus, very desirable to develop electrochemical sensors in which non-aqueous electrolyte systems can be used.